I finally came about to do some distortion measurements I trust on the finished speaker. This one is done at 96 dB @ 1 m, measured at 2 m distance in freefield. Second harmonic peaks at 1% at 1kHz and 7.5kHz which are intrinsic by design with the chosen units. Third harmonic is mostly below 0.1% for all but the lowest octaves, which I'm pleased with.

Thanks Markus, I'm familiar with that paper. As far as I recall the dipole cosine radiation pattern is a fair approximation of Kates findings. But it has been a long time since I looked into the details of that paper.